Create Merge Request unavailable in issues of forked projects
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## Problem to solve
When I create an issue is in a forked project, the "Create merge request" button is not available. I want to be able to create my own issues and then create a merge request.
## Proposal
The "Create merge request" button should be available in the issue, and create a merge request targeting the fork (e.g. the same project that the issue is in)
## Tests
As of 2019-08-05
* Original project: https://gitlab.com/amandarueda/ar-testing-project
* Issue w/ MR option: https://gitlab.com/amandarueda/ar-testing-project/issues/1

* Forked project: https://gitlab.com/amanda-testing-user-group/ar-testing-project
* Issue w/o MR option: https://gitlab.com/amanda-testing-user-group/ar-testing-project/issues/1

## Support Tickets
- https://gitlab.zendesk.com/agent/tickets/31791
- https://gitlab.zendesk.com/agent/tickets/128525
Related links:
cc:/ @MrChrisW @stanhu @jramsay
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